Diffusion pump



Nov. 28, 1967 DIFFUS ION PUMP G. H. BANCROFT Filed March l5, 1965 @Info ai@ Q Q\\n\a i INVENTOR.

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United States Patent C) 3,355,093 DIFFUSION PUMP George H. Bancroft, Rochester, N .Y., assigner, by mesne assignments, to The Bendix Corporation, Detroit, Mich., a corporation of Delaware Filed Mar. 15, 1965, Ser. No. 439,622 1 Claim. (Cl. 230-101) The present invention relates generally to a vapor operated vacuum pump and more particularly to an improved nozzle construction for a vapor diffusion pump.

All diifusion pumps have a certain amount of backstreaming, that is, sorne of the vapor discharged from the nozzle moves towards the inlet of the pump. Backstreaming, of course, reduces the effectiveness of a pump. It is known that the rate of backstreaming can be reduced by surrounding the top nozzle of the pump with baflles or a cold cap. I have found, however, that the rate of backstreaming may be further reduced by controlling the boundary layer on the inner surface of the jet nozzle discharge skirt of at least the first stage nozzle of a diffusion pump.

It is accordingly a principal object of the present invention to reduce the amount of backstreaming in a diffusion pump by providing an improved nozzle construction which provides improved control of the vapor discharge from the nozzle.

Other objects and advantages of the present invention will become apparent from the following detailed description taken in connection with the appended drawings in which:

FIGURE l is a sectional view of a diffusion pump embodying the present invention;

FIGURE 2 is an enlarged view partially in section of the iirst stage nozzle of the pump shown in FIGURE 1;

FIGURE 3 is a top view of the nozzle shown in FIG- URE 2; and

FIGURE 4 is a view similar to FIGURE 2 showing a modification of the present invention.

Referring now to the drawings and more particularly to FIGURE 1, numeral designates a diffusion pump having a pump casing 12 provided with an inlet 14 and an outlet 16. The outlet 16 is adapted to be connected to a suitable forepump (not shown) to bring the diffusion pump outlet pressure to the described operating value. The inlet 14 is adapted to be connected by means of tiange 18 to the vessel to be evacuated.

Pump 10 is provided with a boiler 20` which contains the fluid to be vaporized. Electric heating means 22 is provided to heat the boiler to the desired temperature. A plurality of stacks 24, 26, 28 and 30 are disposed to receive vapor from boiler and to respectively discharge the vapor through nozzles 32, 34, 36 and 38. The nozzles are formed to discharge vapor toward the pump outlet and in a direction whereby the vapor will strike the Walls of casing 12 which are cooled through coils 40. The vapor striking the cooled walls 12 condenses and is returned to the boiler 20.

As best seen in FIGURE 2, the top of first stage nozzle 38 is provided with a discharge skirt 42 or vapor deflecting wall which has a discharge end 44. A cold cap 46 is mounted on a rod 48 so as to surround nozzle 38 in spaced relation thereto. Cold cap 46 is provided with cooling coils 50 which are mounted on side wall 52. The side wall 52 is constructed to extend beyond the discharge end 44 of nozzle skirt 42 and is provided with a plurality of rods or wires 54 extending radially from the lower end thereof. Discharge skirt 42 is provided with a plurality of flow paths that connect the inner surface or interior wall of the skirt 42 with the space 55 between skirt 42 and wall 52 of the cold cap 46. In the embodiment shown in FIG- URES l through 3 the How paths are provided by a belt of foraminous material 56 which forms part of the skirt 42.

In the embodiment shown in FIGURE 4 the ilow paths are provided by slots 58 formed in the wall of skirt 42.

In operation, vapor from boiler 20 iiows through stacks 24, 26, 2S and 30` to be discharged at high velocity from nozzles 32, 34, 36 and 38 away from the pump inlet and in a direction to be condensed on the cooled pump casing Wall 12. Gas molecules which diffuse into the vapor jets are entrained and removed toward the discharge region of the pump. Vapor discharged from the top of first stage nozzle 38 has a tendency to back-stream toward the area of lowest pressure at pump inlet 14. In the prior pumps, the cold cap 46 catches some of the backstreamiug vapor and causes it to condense on the inner surface of the cap. Condensed vapor from cold cap 46 flows down the side walls 42 and onto rod 54 for deposit adjacent the cooled casing wall 12.

In the ordinary diffusion pump, the vapor velocity of the Vapor being discharged from nozzle 38 increases from substantially zero adjacent the walls of skirt 42 to some maximum value near the center of the discharge area. The faster moving vapor near the center of the nozzle tends to turn the slower moving vapor near the skirt walls in a backstreaming direction as it is discharged from the mouth of the nozzle. In the present invention, the tendency to create backstreaming is reduced by providing a plurality of flow paths 56 or 58 in the skirt wall which permit the slow moving vapor to pass through the liow path to be condensed on the cold cap walls. Thus, in a very simple way backstreaming may be substantially reduced by providing boundary layer control along the discharge skirt of the nozzle. The principal benefit of the present invention is obtained by utilizing the boundary layer control on the top of rst stage nozzle only. It is to be understood, however, that the boundary layer control could be applied to all of the nozzles of the pump.

Although the present invention has been described with reference to a specific embodiment it will be readily apparent to one skilled in the art that various rnodiiications and changes may be made without departing from the spirit of the invention.

I claim:

A vapor diffusion pump with la nozzle having a discharge skirt for directing vapor discharge from said nozzle, a cap surrounding said skirt and spaced therefrom, forming a vapor collecting space intermediate said nozzle skirt and said cap, means for cooling said cap to provide a vapor condensing inner surface thereon, and a plurality of flow paths formed in said skirt for removing vapor boundary layer adjacent said skirt, said iiow paths operative to transmit said boundary layer into said vapor collecting space wherein it is condensed on said inner surface.

References Cited UNITED STATES PATENTS 2,501,614 `'3a/1950 Price 230l22 `2,656,096 10/ 1953 Schwarz 230-122 2,934,258 4/1960 Power 230-101 3,000,401 9/1961 Ringleb 230-122 3,067,982 12/ 1962 Wheeler 23o-122 3,165,255 1/ 1965 Landfors 230-101 FOREIGN PATENTS 619,722 3/ 1949 Great Britain.

DONLEY I. STOCKING, Primary Examiner.

W. J. KRAUSS, Assistant Examiner. 

